The present invention relates to a laser used to provide a reference line, and, more particularly, to a laser useful to provide a reference line in applications such as surveying and laying and aligning pipe.
A laser beam has been found to be useful in many surveying applications, and, for example, as a reference line for the predetermined guidance of equipment, either for tunneling, for hydraulic pipe pressing, irrigation work, alignment of railroad tracks and other applications. In these applications, the laser beam is used as a reference line, either on a level or horizontal plane or inclined a predetermined angle from horizontal.
One particularly suitable application for laser alignment systems is for aligning and laying pipeline. For pipe laying applications, a deviation of 1/100 foot per 100 feet due to mounting instabilities is no longer acceptable for runs over 300 feet length. This means that the support of conventional instruments has to be better than 1/1000 inch per foot, or roughly 20 arcseconds. This is not easy to realize and involves sophisticated mounting equipment as well as repeated checking and readjustment.
Heavy equipment moving about the site for the purpose of excavating, laying of the joints, safetying, backfilling, pouring of concrete for manholes and bases, etc., with the associated vibration and shifting of large weights causes the pipeline to settle and move. The corresponding angular motion of a specific joint can typically range from a few seconds to minutes of arc, depending on the specific conditions.
If the grade instrument is anchored to a particular joint of pipe, for reasons of maximum mechanical rigidity, these angular variations of the position of the joint will cause proportionate wandering of the laser beam, of up to several hundreds of a foot per 100 feet distance. Similarly, instruments mounted on tripods or other means of mechanical support, with a separating layer of soil or fill between their base and the pipe-line, will be subject to settling and wandering of the reference axis, causing similar effects and errors.
In the prior art systems, frequent readjustment is required. During the operation of one prior art device, drifts from 20 to 60 seconds were typically observed over periods of an hour, depending on the specific job situation.